Answer:
The Civil War Siege of Petersburg lasted from June 15, 1864 to April 2, 1865. Ulysses S. Grant's northern armies (the Army of the Potomac and the Army of the James) launched nine offensives against Robert E. Lee's Army of Northern Virginia and assorted troops from the Department of Southern Virginia and North Carolina. Lee's Confederate troops were defending the cities of Richmond and Petersburg.
Richmond, as the capital of the Confederacy, was an incredibly important symbol of the Confederacy. Petersburg, 20 miles south, supplied Richmond through several railroads coming in from other parts of the South.
Interestingly, the Siege of Petersburg was not a true siege because the Union army never completely encircled Lee's Confederates, who could and ultimately did leave to the west when Lee's lines were irreparably broken on April 2, 1865.
Over 50,000 Union soldiers and 32,000 Confederate soldiers were casualties in the nine month campaign.